1. Field of the Invention
The present invention relates to image forming apparatuses, and control methods and control programs thereof.
2. Description of the Related Art
Hitherto, image forming apparatuses have been used in which image forming is carried out by irradiating a laser light onto a photosensitive drum. When using a laser optical system in this manner, due to causes such as error in mechanical precision of optical components or fitting during assembly and installation for example, there are some problems. For example, the start position of the scanning line of the laser light scanned on the photosensitive drum was displaced. Furthermore, a desired magnification ratio may not be achieved, the scanning line may be tilted, curved, or distorted on the photosensitive drum. In regard to writing position and magnification ratio corrections among these, techniques have hitherto been employed involving detecting error and performing corrections electrically. However, since electrical corrections of this manner are difficult for tilt or warp in a scanning line, conventionally corrections have been carried out using high quality optical components so that tilt and warp are not produced. Thus, an apparatus having an expensive configuration is necessitated and moreover precise fine adjustments are necessary during assembly, which leads to increased man-hour, thereby resulting in higher production costs.
In regard to these issues, Japanese Patent Laid-Open No. 2004-170755 (page 13, FIG. 1) proposes a method in which registration is detected at a plurality of points in at least three or more locations in a main scanning direction, and the image data is changed so as to correct warp such as tilt or curving in the main scanning direction, which has been calculated from the detected registration. Here, in relation also to corrections of one pixel or less in the sub-scanning direction, writing positions are detected from a result of detecting registration in a same manner, and the image data is changed so as to correct the sub-scanning direction writing positions that have been detected. By performing image forming with image data that has been changed in this manner, positional corrections can be carried out on tilt or warp in the scanning line without using expensive optical components or performing precise adjustment processes, thereby enabling a high image quality image forming apparatus to be provided at low cost.
Unfortunately, in a case of a configuration indicated in the aforementioned patent document, it is necessary to provide a plurality of registration detection unit for at least three locations in the main scanning direction, and thus there is a problem in that the apparatus itself greatly increases in cost.
Moreover, although correction is carried out on the image data by deriving tilt/warp in the scanning line from a result of detecting the registration pattern, there are cases where the correction positions are roughly detected in the main scanning direction depending on the form of the tilt and warp in the scanning line, and corrections cannot be carried out optimally.